Deep structure of the northeastern Japan arc and its implications for crustal deformation and shallow seismic activity

Transcription

1 Tectonophysics 403 (2005) Deep structure of the northeastern Japan arc and its implications for crustal deformation and shallow seismic activity Akira HasegawaT, Junichi Nakajima, Norihito Umino, Satoshi Miura Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai , Japan Received 16 September 2004; received in revised form 18 March 2005; accepted 29 March 2005 Available online 10 May 2005 Abstract Seismic tomography studies in the northeastern Japan arc have revealed the existence of an inclined sheet-like seismic lowvelocity and high-attenuation zone in the mantle wedge at depths shallower than about 150 km. This sheet-like low-velocity, high-attenuation zone is oriented sub-parallel to the subducted slab, and is considered to correspond to the upwelling flow portion of the subduction-induced convection. The low-velocity, high-attenuation zone reaches the Moho immediately beneath the volcanic front (or the Ou Backbone Range) running through the middle of the arc nearly parallel to the trench axis, which suggests that the volcanic front is formed by this hot upwelling flow. Aqueous fluids supplied by the subducted slab are probably transported upward through this upwelling flow to reach shallow levels beneath the Backbone Range where they are expelled from solidified magma and migrate further upward. The existence of aqueous fluids may weaken the surrounding crustal rocks, resulting in local contractive deformation and uplift along the Backbone Range under the compressional stress field of the volcanic arc. A strain-rate distribution map generated from GPS data reveals a notable concentration of east west contraction along the Backbone Range, consistent with this interpretation. Shallow inland earthquakes are also concentrated in the upper crust of this locally large contraction deformation zone. Based on these observations, a simple model is proposed to explain the deformation pattern of the crust and the characteristic shallow seismic activity beneath the northeastern Japan arc. D 2005 Elsevier B.V. All rights reserved. Keywords: Arc magmatism; Aqueous fluids; Crustal deformation; Shallow seismicity; Subduction zone; Northeastern Japan arc 1. Introduction Northeastern Japan is located at a subduction zone, where the Pacific plate subducts downward into the T Corresponding author. Tel.: ; fax: address: hasegawa@aob.geophys.tohoku.ac.jp (A. Hasegawa). mantle at a convergence rate of 8 9 cm/year and at an angle of about 308. Many shallow earthquakes occur beneath the Pacific Ocean mainly along the upper boundary of the Pacific plate associated with its subduction. Beneath the land area, shallow earthquakes also occur in the upper crust; many of them are concentrated in a long, narrow zone extending along the volcanic front or the central mountainous range (Ou Backbone Range) which runs through the /$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi: /j.tecto

2 60 A. Hasegawa et al. / Tectonophysics 403 (2005) middle of the land area nearly parallel to the trench axis (Fig. 1). Great progress has been made in the last few years in understanding the stress concentration mechanism causing interplate earthquakes beneath the Pacific Ocean off the northeastern Japan arc. Asperities are distributed in patches surrounded by stable sliding areas on the plate boundary. Aseismic slip in the surrounding stable sliding areas results in the accumulation of stress at the asperities, and earthquakes occur when the strength limit of an asperity is reached leading to sudden slip. It has gradually become clear that this kind of asperity model (Lay and Kanamori, 1981) represents an accurate description of the mechanism of such earthquakes (Nagai et al., 2001; Yamanaka and Kikuchi, 2004; Matsuzawa et al., 2002; Okada et al., 2003, Hasegawa et al., in press). Understanding the mechanism of stress concentration that leads to shallow inland earthquakes (intraplate earthquakes) in the arc crust, on the other had, has advanced more slowly. Why, of the many active faults, does stress concentrate along just one of them, leading to slip and an earthquake? It is to be expected that once slip occurs on an active fault, producing an earthquake, stress would become concentrated in regions adjacent to extensions of the fault, but in general, inland earthquakes occur in isolation, and related earthquakes in adjacent regions are rarely if ever observed. Why is this so? Our current level of Fig. 1. Map showing the northeastern Japan arc and its surroundings. Red triangles and thick gray line denote active volcanoes and the volcanic front, respectively. White arrow indicates the direction of the relative plate motion (Demets et al., 1994). The bathymetry is taken from the Japan Coast Guard. 1. Iwate volcano, 2. Naruko volcano.

3 A. Hasegawa et al. / Tectonophysics 403 (2005) understanding is not sufficient to explain these facts. It is clear that this scenario cannot be explained by a simple model in which an elastic upper crust supports stress caused by relative plate motion, with slip (and hence earthquakes) occurring when the stress exceeds the strength of the fault surface as a plane of weakness within the crust (Iio, 1996, 1998). Recent seismic tomography studies in the northeastern Japan arc have provided new information that shows that water supplied by dehydration of the subducting slab reaches the upper crust via the mantle wedge, entrained in an upwelling flow in the mantle that travels nearly parallel to the slab as a seismic lowvelocity, high-attenuation zone in the mantle wedge. The sheet-like upwelling flow aligned nearly parallel to the slab reaches the Moho near the Backbone Range (or the volcanic front). Consequently, partial melting is widely distributed along the volcanic front immediately below the Moho. When the molten material in such a melting zone approaches the surface, it cools and partially solidifies, expelling water contained in the molten material. It is expected that this water migrates to even shallower levels. Seismic tomography provides images of the upwelling paths of water in the upper crust as the low-velocity zones. The result is the continuous supply of water expelled from the subducting slab into a region below the Backbone Range. Research on surface deformation based on GPS data has revealed a zone of strain concentration that extends north south along the Backbone Range, representing the local predominance of contractive deformation in the direction of relative plate motion along the Backbone Range. This zone of strain concentration is located above where the upwelling flow in the mantle wedge reaches the Moho. The concentrated supply of water originating from the slab must weaken the crustal material, causing contractive deformation to occur locally, that is, anelastic deformation occurs locally even within the upper crust. It is inferred that since this anelastic deformation is non-uniform in space, shallow inland earthquakes serve as a mechanism for making the overall deformation more uniform. Based on the present data, we propose this model of stress concentration mechanism as a model for the occurrence of shallow inland earthquakes in the northeastern Japan arc. 2. Mantle wedge structure of the northeastern Japan arc Nakajima et al. (2001a,b), using data from the seismic observation network, the density of which has recently been increased, calculated the three-dimensional seismic wave velocity structure for the northeastern Japan arc, updating the results of Zhao et al. (1992). Figs. 2 and 3 show the P-wave velocity (Vp) and S-wave velocity (Vs) on cross-sections perpendicular to the island arc. In any of the vertical crosssections (a) to (f), the Pacific Plate subducting beneath the arc is imaged as a strong high-vp and high-vs region. Within the mantle wedge immediately above the Pacific Plate, low-vp, low-vs regions inclined nearly parallel to the slab and extending from depths of about 100 to 150 km to the Moho appear clearly. These regions of low seismic wave speed appear clearly not only in cross-sections (a), (b), (d) and (f), which pass through active volcanoes, but also in cross-sections (c) and (e), which do not include any volcanoes. This illustrates the existence of a single sheet-like low-velocity zone inclined nearly parallel to the slab within the mantle wedge. This low-velocity zone has high Vp/Vs values. Fig. 4 shows distribution of Vp/Vs ratio at a depth of 40 km. We can see that a high Vp/Vs (and low Vp, Low Vs) zone is distributed along the volcanic front immediately below the Moho. Similar low-velocity zones inclined nearly parallel to slabs have also been observed in mantle wedges in other subduction zones (Abers, 1994; Zhao et al., 1995, 1997; Gorbatov et al., 1999), although none are as clear as those in northeastern Japan (Figs. 2 and 3). Seismic attenuation structure provides additional information on the physical states of the earth s interior. Three-dimensional P-wave attenuation structure beneath NE Japan was estimated by a joint inversion for source parameters, site response and Qp values (Tsumura et al., 2000). Fig. 5 shows across-arc vertical cross-sections of Qp values along three lines in the inserted map. Low Qp (high attenuation) zones inclined nearly parallel to the slab are clearly seen for all the cross-sections, although the extent of drop in Qp-value is not large for cross-sections A and B. The low-qp zones are consistent with the inclined low-v zone in Figs. 2 and 3. Thus there exists an inclined sheet-like low Vp, low Vs, high Vp/Vs and low Qp

4 62 A. Hasegawa et al. / Tectonophysics 403 (2005) Fig. 2. Across-arc vertical cross-sections of P-wave velocity perturbations along lines in the inserted map of NE Japan (Nakajima et al., 2001a). The solid line and red triangles at the top represent land area and active volcanoes, respectively. Open and red circles denote earthquakes and deep, low-frequency microearthquakes, respectively. zone in the mantle wedge beneath the northeastern Japan arc. 3. Upwelling flow within the mantle wedge We infer that the inclined sheet-like low-v and low-q zone described above corresponds to the upwelling flow in the secondary convection (McKenzie, 1969) accompanying slab subduction. Since temperature increases with depth, the interior of this upwelling flow is at a higher temperature than the surrounding region, and as such should have lower viscosity. In an old plate subduction zone such as northeastern Japan, water supplied from dehydration of the subducted slab may form a temporary layer of serpentine and chlorite in the mantle wedge immediately above (Davies and Stevenson, 1992; Iwamori, 1998), which is then dragged downward to a depth of km where dehydration decomposition occurs (Iwamori, 1998; Schmidt and Poli, 1998). Slightly low velocity areas are imaged immediately above the subducted slab in some of vertical cross-sections of Figs. 2 and 3 (e.g., Fig. 3(b), (d), which might correspond to this temporary layer of serpentine and chlorite, although more studies with much higher resolutions are required to confirm it. The water released by this dehydration at depth is then transported upward, encountering the upwelling flow at depths of km. The supply of water to the upwelling flow has the effect of lowering the solidus temperature. From a comparison of the seismic wave attenuation structure described in the previous section (Tsumura et al., 2000) with laboratory experiment data, the temperature within the low-v, low-q zone is estimated to be higher than that of the peridotite wet

5 A. Hasegawa et al. / Tectonophysics 403 (2005) Fig. 3. Across-arc vertical cross-sections of S-wave velocity perturbations along lines in the inserted map (Nakajima et al., 2001a). Other symbols are the same as in Fig. 2. solidus (Nakajima and Hasegawa, 2003a). Further, Nakajima et al., in press inferred from the ratio of falloff rates of P-wave and S-wave velocities that melt inclusions are included in the low-v, low-q zone, having aspect ratios of and volume fractions of 0.1 to several percent. The existence of such a low-velocity zone inclined nearly parallel to the slab at depths of less than 150 km, as detected by seismic tomography, has also been confirmed by numerical simulation of the secondary convection that accompanies plate subduction. Eberle et al. (2002) performed a numerical simulation of the corner flow that accompanies plate subduction using a temperature-dependent viscosity coefficient, and found that a low-velocity zone with velocities several percent slower than in the surrounding region was generated, which would correspond to the present upwelling region. The low-velocity zone determined by Eberle et al. (2002) was aligned nearly parallel to the slab, was separated from the upper surface of the slab by about 50 km, and extended to depths of no more than 125 km, accurately reproducing the lowvelocity zone observed in northeastern Japan (Figs. 2 and 3). The inferred water transport paths in the northeastern Japan subduction zone are shown schematically in Fig. 6(a). The upwelling of hot mantle material from depth and the addition of water may cause partial melting with a volume fraction on the order of 0.1 to several percent. Melt is formed both by decompression melting and melting due to water addition. From the fact that the inclined low-velocity zone is only clearly observed at depths shallower than about 150 km (Zhao and Hasegawa, 1993), it is inferred that melting by the addition of water plays an important role in melt formation. Thus, water that originated from the slab is eventually incorporated into the melt. The upwelling flow including this melt eventually

6 64 A. Hasegawa et al. / Tectonophysics 403 (2005) Depth = 40 km fast directions in the back-arc region are nearly parallel to the direction of relative plate motion. Most of stations with such trench-perpendicular directions are located above the inclined low-velocity zone (i.e. upwelling flow) in the mantle wedge. The observed trench-perpendicular fast directions would be explained by lattice preferred orientation of minerals caused by flow-induced strain in the mantle wedge (Ribe, 1992; Tommasi, 1998; Zhang and Karato, 1995). On the contrary, trench-parallel fast directions are seen in the fore-arc region. Perhaps another mechanism is working to cause these directions in the fore-arc mantle wedge. Seismic tomography research is also providing important information on the variation of magma Vp/Vs Fig. 4. Vp/Vs ratio at a depth of 40 km (Nakajima et al., 2001a). Red triangles denote active volcanoes. reaches the Moho immediately below the volcanic front, resulting in the accumulation of large amounts of melt immediately below the Moho along the volcanic front. Seismic tomography clearly reveals this continuous distribution of partially molten material along the volcanic front and immediately below the Moho as a region of low Vp, low Vs, high Vp/Vs and low Qp (Figs. 2 through 5). From this point of view, the volcanic front can be regarded to form where a sheet-like upwelling flow in the mantle wedge reaches the Moho. Seismic anisotropy structure beneath the arc, shown in Fig. 7 (Nakajima and Hasegawa, 2004), seems to support the existence of this upwelling flow in the mantle wedge. Fig. 7 clearly shows a systematic spatial variation in directions of fast shear-waves. The Fig. 5. Across-arc vertical cross-sections of P-wave attenuation structure along lines in the inserted map (Tsumura et al., 2000). Red and blue colors represent high and low attenuations, respectively, according to the scale at the bottom. Other symbols are the same as in Fig. 2.

7 A. Hasegawa et al. / Tectonophysics 403 (2005) Fig. 6. (a) Schematic diagram of vertical cross-section of the crust and upper mantle of NE Japan, showing the inferred transportation paths of aqueous fluids. (b) Schematic 3D structure of the crust and upper mantle of NE Japan showing the upwelling flow with varying thickness in the mantle wedge. formation along the island arc. Recently, Tamura et al. (2002) investigated the distribution of Quaternary volcanoes in northeastern Japan, and found that the volcanoes are distributed in long and narrow bands perpendicular to the island arc, forming 10 clusters of volcanoes occupying an average width of 50 km. These cross-arc bands in which Quaternary volcanoes are concentrated coincide with regions of elevated topography and low Bouguer gravity anomaly. Tamura et al. (2002) concluded that volcanoes form where inclined hot fingers (upwelling regions) distributed across a width of 50 km in the mantle wedge

8 66 A. Hasegawa et al. / Tectonophysics 403 (2005) Velocity perturbation (%) Delay time (sec) Fig. 7. Direction of fast shear-wave and delay time plotted at each station superposed on shear-wave velocity perturbations in the mantle wedge (Nakajima and Hasegawa, 2004). Black lines denote the direction of fast shear-wave and length is proportional to the average time delay between the leading and following shear-waves. Velocity image is the shear-wave velocity perturbations along the inclined low-velocity zone in the mantle wedge as in Fig. 8(a). Red triangles show active volcanoes. White arrow indicates the direction of the relative plate motion (Demets et al., 1994). at a depth of 150 km reach the surface. The repeated supply of magma from hot fingers in the mantle wedge to the crust immediately above causes the bedrock to be uplifted and Quaternary volcanoes to form. They further concluded that the magma that is supplied accumulates beneath the Moho, producing the local low Bouguer gravity anomalies. To confirm the model of Tamura et al. (2002), we attempt to image the low-velocity zone in the mantle wedge with a higher spatial resolution (Hasegawa and Nakajima, 2004). In this study, the velocity structure outside of the mantle wedge was fixed to that obtained earlier by Nakajima et al. (2001a), and the velocity distribution within the mantle wedge was estimated using the same data set. The spatial resolution was 10 km or finer in both the horizontal and depth directions. The distribution of S-wave velocity obtained is shown in Fig. 8(a). The figure shows the S-wave velocity perturbations taken along the inclined low-velocity zone. The value is that along the surface of minimum S-wave velocity within the mantle wedge, and thus the figure shows the distribution of S-wave velocity perturbations along the curved surface joining the core of the low-velocity zone. As Tamura et al. (2002) predicted, the extent of velocity drop within the low-velocity zone varies clearly along the strike of the island arc. Comparing these results with the topographic map (Fig. 8(b)), we can see that there is good agreement between the regions where the velocity drop is locally particularly strong in the low-velocity zone distributed from 30 to 150 km depth in the mantle wedge and the regions where elevations in the topography are high from the Backbone Range to the back-arc region. Quaternary volcanoes (red circles) are distributed in those regions. In addition, low-frequency microearth-

9 A. Hasegawa et al. / Tectonophysics 403 (2005) Fig. 8. (a) S-wave velocity perturbations along the inclined low-velocity zone in the mantle wedge of NE Japan. (b) Topography map of NE Japan. Deep low-frequency microearthquakes were located by the Japan Meteorological Agency and Okada and Hasegawa (2000). Thick lines denote active faults (Active Fault Research Group, 1991). quakes (white circles) produced at depths of km, believed to be caused by sudden movements of fluids in the crust (Hasegawa et al., 1991; Hasegawa and Yamamoto, 1994), are seen to occur immediately above zones of particularly large velocity drop in the mantle wedge. Spatial correlations between the following features can be clearly seen in Fig. 8: 1) Regional variation of low-velocity zone distributed from 30 to 150 km depth in the mantle wedge, 2) The distribution of low-frequency microearthquakes occurring at km depth, 3) The distribution of Quaternary volcanoes at the surface, 4) The distribution of topographical elevations extending from the Backbone Range toward the back-arc region. The structure of the crust and upper mantle in northeastern Japan, and the upwelling flow in the mantle, as inferred based on these observational facts, are shown schematically in Fig. 6(b), which shows a three-dimensional expansion of the two-dimensional cross-section in Fig. 6(a). The upwelling flow in the mantle wedge is sheet-like, with a thickness that varies locally from place to place, rather than occurring in fingers as suggested by Tamura et al. (2002). The volcanic front is formed where this upwelling flow finally contacts the Moho. As the flow approaches the Moho, it slows down. The melt contained in the flow accumulates over a wide area along the volcanic front, immediately below the Moho, resulting in the low-velocity, high-attenuation zone that is seen to extend over a wide areas along the volcanic front. Seismic tomography has revealed that in the volcanic zones, differentiation occurs and magma rises to the middle crust (see Figs. 12 and 13).

10 68 A. Hasegawa et al. / Tectonophysics 403 (2005) Within the sheet-like upwelling flow, in regions of the back-arc side where the sheet is locally thick and there is a large amount of melt, part of the melt sometimes separates from the upwelling flow before it reaches the Moho along the volcanic front. According to the estimate by Nakajima et al., in press obtained using the rates of decrease of Vp and Vs, the volume fractions of melt within these regions of the back-arc side in the upwelling flow are on the order of 0.1% to several percent. The separated melt rises straight upward in the plumes, and accumulates beneath the Moho. Part of it continues to rise and penetrates into the crust, forming volcanoes and uplifting the bedrock. We infer that this is how the concentrations of Quaternary volcanoes and elevations of topography extending from the volcanic front toward the back-arc region formed, and probably this formation process continues today. The alternation of the regions where Quaternary volcanoes are concentrated and regions without volcanoes in the direction along the island arc is presumed to be due to the variation of partial melting in the upwelling flow in the mantle wedge at depths from 30 to 150 km along the island arc. analysis software. Using this feature, the coordinates of an isolated observation point can be estimated from the observation data for that point alone, without having to form a baseline. For this estimation, parameters estimated in advance to high precision by JPL including orbital histories of GPS satellites, clock errors and the Earth s rotation are used. Data were analyzed using this Precise Point Positioning (PPP) technique (Zumberge et al., 1997). East west components of horizontal strain rates estimated from observational data from January 1997 to December 2001 are shown in Fig. 9. Constraints have been applied so as to ensure that the strain rate is continuous in space (Miura et al., 2002; Sato et al., 2002). The east west components are shown as the direction in which the deformation that accompanies the plate convergence predominates; north south /yr Zones of concentrated deformation along the Backbone Range Observational data on the surface deformation field obtained from the nationwide GPS continuous observation network (GEONET) of the Geographical Survey Institute of Japan have provided a great deal of information that was previously impossible to obtain, such as that related to the temporal and spatial variations of interplate coupling at plate boundaries. Suwa et al. (2003) and Sato et al. (2002) have analyzed data from the GEONET and the observational network of Tohoku University from 1997 to 2001 seeking to clarify surface deformation in the Tohoku region. GIPSY-OASIS II (GPS Inferred Positioning System-Orbit Analysis and Simulation Software II), developed by the Jet Propulsion Laboratory (JPL) of the American National Atmospheric and Space Administration (NASA) was used for GPS data analysis. This analysis software estimates parameters such as clock drift in satellites and receivers as probability variables without the need to take double phase differences. This is a big advantage over other Fig. 9. Distribution of horizontal east west strain rate estimated from GPS observations for the period from 1997 to 2001 (Sato et al., 2003). Contour interval is 100 ppb/year. Red triangles denote active volcanoes

11 A. Hasegawa et al. / Tectonophysics 403 (2005) components are much smaller than east west components. It can be seen from Fig. 9 that there is a beltlike zone in which contractive deformation is concentrated along the Backbone Range (or the volcanic front). The concentrated zone in which contractive deformation predominates in the direction of relative plate motion is therefore distributed in a long and narrow band that runs throughout Tohoku along the Backbone Range /yr. 5. Deformation of the arc crust and shallow inland earthquakes their relationship with fluids As shown in Section 3, the inclined sheet-like upwelling flow in the mantle wedge reaches the Moho along the volcanic front, that is, the Backbone Range. The distribution of the Vp / Vs ratio immediately below the Moho is shown in Fig. 4. The upwelling flow, imaged as low-vp, low-vs, high-vp/ Vs and low-qp regions, is distributed nearly continuously along the volcanic front, immediately below the Moho. The melt incorporated into the upwelling flow either butts up against the bottom of the crust or penetrates into the crust. When it cools in the crust and partially solidifies, water is expelled from it and moves upward. Thus, water of slab origin is supplied continuously to the shallow part of the crust along the Backbone Range. The presence of water is consistent with the concentration of low-frequency microearthquakes (Hasegawa et al., 1991; Hasegawa and Yamamoto, 1994) at depths near the Moho, and with S-wave reflectors at intermediate crustal depths (Hori et al., 2004) along the Backbone Range. The presence of water can be expected to weaken the crustal material and to produce local contractive deformation under a compressive stress field. We infer that this happens in the concentrated deformation zone along the Backbone Range as shown in Fig. 9. This concentrated deformation zone is also the location of considerable present microearthquake activity, as shown in Fig. 10. The deformation pattern of the arc crust in northeastern Japan inferred from these observed facts is schematically shown in Fig. 11(a). As melt cools and solidifies, water that have separated from the melt sometimes moves suddenly in the lower crust, and is observed as deep low-frequency microearthquakes in Fig. 10. Distribution of horizontal east west strain rate for the period from 1997 to 2001 (Sato et al., 2003), and shallow earthquakes located by the seismic network of Tohoku University for the same period. the lowermost crust (Hasegawa et al., 1991; Hasegawa and Yamamoto, 1994). The water forms a sill at intermediate crustal depths and accumulates, perhaps corresponding to the bright S-wave reflectors that have been detected across a wide area along the Backbone Range (Matsumoto and Hasegawa, 1996; Hori et al., 2004). In the Backbone Range, the temperature is locally increased by the infiltration of high-temperature material from the upper mantle, and the bottom of the seismogenic layer (the boundary between brittle and ductile layers) is locally elevated (Hasegawa and Yamamoto, 1994; Hasegawa et al., 2000). Corresponding to this, the observed crustal heat flow has locally high values in the Backbone Range (Furukawa, 1993; Tanaka and Ishikawa, 2002). The water continues to rise and reaches the upper crust, causing plastic deformation in some part of the brittle upper crust.

12 70 A. Hasegawa et al. / Tectonophysics 403 (2005) (a) Backbone Range WEST seismogenic zone contraction & uplift (partly anelastic deformation) small earthquakes EAST large earthquake brittle to ductile transition lower crust Moho upper mantle low-v low-v low-q S-wave reflectors low-f events upwelling flow (b) Backbone Range elastic deformation partly anelastic deformation elastic deformation large contraction reverse fault small contraction reverse fault large contraction Fig. 11. (a) Schematic illustration of across-arc vertical cross-section of the crust and uppermost mantle, showing the deformation pattern of the crust and the characteristic shallow seismic activity beneath NE Japan. (b) Map view schematically showing the deformation pattern of the upper crust. In the Backbone Range, where the seismogenic layer is locally thin and melt and water are distributed in the lower crust, the entire crust will be locally weak in comparison with the surrounding region. For this reason, the arc crust, which is being compressed in the direction of relative plate motion, deforms elastically outside of the Backbone Range, but anelastically in part within the upper crust along the Backbone Range, which can be expected to cause local contraction and uplift. We infer that the concentrated deformation region shown in Fig. 9 was formed in this way, although some extension is observed locally around Iwate volcano, which is probably related to the volcanic activity of Mt. Iwate, which started in Numerical simulation studies are essential to obtain a quantitative model having spatial perturbations of elastic and viscous rheological constants which can explain the observed amount of the deformation, however, they are left for future studies. Research on surface deformation based on analysis of GPS data (Sato et al., 2003) is steadily revealing

13 A. Hasegawa et al. / Tectonophysics 403 (2005) evidence of uplifting zones along the Backbone Range as predicted by the present model. Local contractive deformation along the Backbone Range is perhaps caused by asseismic slip on the deep extension of faults and/or by plastic volume deformation in the lower crust, leading to stress concentration in the upper crust immediately above. Anelastic deformation may also occur partially, in the upper crust. This eventually leads to the rupture of the whole upper crust, producing a shallow inland earthquake that makes the deformation uniform in space (Iio et al., 2000, 2002). Anelastic contractive deformation along the Backbone Range including the upper crust causes numerous shallow microearthquakes as it advances, as seen in Fig. 10. Fig. 9 shows that, there is one more long, narrow region on the fore-arc side where contractive deformation predominates, in addition to the Backbone Range where the upwelling flow in the mantle wedge reaches the Moho. This region, in northern Miyagi Prefecture and southern Iwate Prefecture, also has a concentration of shallow microearthquakes (Fig. 10). This region includes the hypocenters of the 1900 Northern Miyagi earthquake (M7.0) and the 1962 Northern Miyagi earthquake (M6.5). Vp, Vs and Vp/ Vs ratios in east west vertical cross-section along a line across this region are shown in Fig. 12. In this region, a large amount of data is available from densely spaced temporary observation stations, making imaging possible at higher spatial resolution (Nakajima and Hasegawa, 2003b). The cause of the concentrated deformation zone on the fore-arc side, which could not be understood from the image immediately below the Moho (Fig. 4), can perhaps be understood from Fig. 12. In addition to the lowvelocity zone extending from below the Moho beneath the Backbone Range to immediately below the Naruko volcano, there is another low-velocity zone that branches off and extends to the eastern side (the fore-arc side). Depth (km) Depth (km) Naruko volcano E E (a) (c) Vp/Vs dvp Vp/Vs (d) Depth (km) Naruko volcano R2 bright S-wave reflectors low-f microearthquakes + + C1 R1 + + C3 Distance (km) + C2 1 Mag. 5 Electrical Resistivity dv(%) Fig. 12. EW vertical cross-sections of (a) P-wave and (b) S-wave velocity perturbations, (c) Vp/ Vs (Nakajima and Hasegawa, 2003b), and (d) electrical resistivity (Mitsuhata et al., 2002) along line a in Fig. 10. Rectangles in (a), (b) and (c) show the range of cross-section in (d) in both horizontal and vertical directions. Red circles and dots denote low-frequency microearthquakes and shallow earthquakes, respectively. Red lines indicate S-wave reflectors (bright spots) (Hori et al., 2004), and red triangles on the top denote active volcanoes. Open circles in (d) indicate shallow earthquakes. (b) dvs

14 72 A. Hasegawa et al. / Tectonophysics 403 (2005) Nakajima and Hasegawa (2003b) inferred from the rates of decrease of Vp and Vs that about 1% melting occurs in the upper mantle, while several percent melting occurs in the lower crust, with about 0.3 5% water in the upper crust in this low-velocity zone. An MT survey conducted in the hypocentral region of the 1962 Northern Miyagi earthquake detected a clear low-resistivity zone in almost exactly the same location as the low seismic velocity zone as shown in Fig. 12(d) (Mitsuhata et al., 2002). Immediately above this zone there is a sheet-like distribution of microearthquakes, dipping to the west, representing aftershocks of the 1962 Northern Miyagi earthquake (Kono et al., 1993). From these observations, we infer that water of slab origin is supplied not only to the Backbone Range but also to the hypocentral region of the 1962 Northern Miyagi earthquake on the fore-arc side. It is conceivable that this causes local contractive deformation in this region as well as in the Backbone Range. In the location where contractive deformation occurs locally, not only are microearthquakes concentrated (Fig. 10), but large earthquakes that rupture the entire seismogenic layer also occur. We infer that contractive deformation occurs principally as anelastic deformation where the entire crust including the upper crust has been locally weakened. The observations given below suggest that since this kind of anelastic deformation does not proceed uniformly in space, large earthquakes that cause the overall contractive deformation to become uniform occur at locations of smaller contractive deformation. Fig. 13 shows the Vp/Vs ratio on a vertical crosssection along the Backbone Range. Regions of high Vp/Vs ratio, believed to be regions of partial melting, are distributed immediately beneath two volcanic areas, in the north and south, reaching intermediate crustal depths. In these two areas, the amount of melt supplied from the mantle wedge, and consequently the amount of water, must be greater than the area between the two areas. Accordingly, within these two areas, it can be expected that weakening of the crust will be considerable and that local contractive deformation will proceed rapidly. If this is the case, then stress will be concentrated in the area between these two areas, perhaps causing a reverse fault earthquake to occur at the edge of the Backbone Range (or inside it), as shown schematically in Fig. 11(b). In fact, the fault plane of some large earthquakes such as the 1896 Rikuu earthquake (M7.2) was not within these two volcanic areas, but at the western and eastern edges of the area (or inside it) between them (Active Fault Research Group, 1991). Even within the volcanic area, it appears that the same kind of phenomenon is taking place, although on a smaller scale. Fig. 14 shows the S-wave velocity distribution at a depth of 4.5 km in the Onikobe area of northern Miyagi Prefecture (Onodera et al., 1998), which is part of the above-mentioned volcanic area. In this area, the lower boundary of the seismogenic layer (the brittle to ductile transition zone) is relatively shallow, on the order of 7 km (Hasegawa et al., 2000). The estimated velocity distribution shows that the velocity within the caldera is low while that outside the caldera is high. It is expected that more water is Depth (km) A Mt. Naruko Mt. Kurikoma Distance (km) Vp/Vs Mt. Akitakoma B bright S-wave reflectors low-f microearthquakes B A Fig. 13. NS vertical cross-section of Vp/Vs structure in NE Japan along the line in the inserted map (Nakajima et al., 2001b). Other symbols are the same as in Fig. 12.

15 A. Hasegawa et al. / Tectonophysics 403 (2005) Fig. 14. S-wave velocity perturbations at 4.5 km depth (Onodera et al., 1998) and fault planes of earthquakes (Umino et al., 1998) in the Onikobe area shown in the inserted map. Fault planes of earthquakes with magnitudes greater than ~5 are indicated by rectangles. Arrows in each fault plane show slip vectors. Small circles denote aftershocks of the M5.9 Onikobe earthquake sequence in Caldera rims are indicated by bold lines (Yoshida, 2001), and red triangles denote active volcanoes. supplied within the caldera than outside the caldera, and consequently there will be considerable anelastic contractive deformation within the caldera. In 1996 there was considerable seismic activity in this region, with the largest earthquake a M5.9 event. In this region, where the seismogenic layer is locally thin, and at depths of around 7 km, the M5.9 earthquake was sufficient to rupture the entire seismogenic layer (Umino and Hasegawa, 2002). From Fig. 14, relatively large earthquakes for this region (M5 class) occur not inside the calderas, but around them. In particular, the M5.9 earthquake occurred between the Sanzugawa caldera and the Onikobe caldera. Thus, the M5.9 earthquake occurred in the region between the calderas to compensate for the delay in the progress of anelastic contractive deformation. This phenomenon is similar to that shown schematically in Fig. 11(b), but on a smaller scale. 6. Concluding remarks In the northeastern Japan arc, shallow earthquakes are concentrated in a region of large contractive deformation in the direction of relative plate motion. Research based on a comparison of crustal horizontal deformation rates over the last 100 years has previously confirmed that such a region also corresponds to a region of low seismic velocity (Hasegawa et al., 2000). Based on these observations, Hasegawa et al. (2000) inferred the upwelling of water from depth to weaken the crust and increase local crustal

16 74 A. Hasegawa et al. / Tectonophysics 403 (2005) contraction rates, resulting in shallow crustal earthquakes in such areas. In the present paper, this concept was extended, and a simple model was proposed based on the highresolution three-dimensional velocity structure determined by seismic tomography and detailed crustal deformation determined by GPS. The model facilitates our understanding of the processes of deformation in the island arc and the occurrence of shallow inland earthquakes in northeastern Japan. Although the validity of this model must await future verification, at least water plays an important role in crustal deformation and in the occurrence of shallow inland earthquakes. Acknowledgments We wish to express our thanks for comments by H. Iwamori, Y. Iio, T. Matsuzawa, and two anonymous reviewers which have contributed importantly to improve this paper. 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